A positive selection procedure for strains with mutations affecting glutamine synthetase activity has been developed in Salmonella typhimurium. This and other selection procedures are being used to isolate strains with mutations in the glutamine synthetase structural gene and in genes for regulatory controlling glutamine synthetase activity. Particular attention will be paid to strains with mutations affecting covalent modification of the enzyme by adenylylation- deadenylylation. During the next year we will (1) characterize regulatory effects caused by mutations in the glutamine synthetase structural gene (glnA), and (2) try to develop selection procedures for strains with mutations affecting the adenylylating-deadenylylating enzymes.